Printing screens for screen printing textiles are typically made of perforated cylindrical sleeves of nickel having a thickness of 0.1 mm, a diameter of between 200 and 400 mm and a length of one to three meters.
Conventionally such screens are coated with a photosensitive lacquer which covers the outer surface of the screen and fills all of its perforations. To produce an ink transmitting pattern the lacquer is exposed to ultra-violet light through a transparent sheet carrying an opaque pattern which is usually a negative resulting from a photographic process. Portions of the photosensitive lacquer which are exposed to ultra-violet light remain after development of the lacquer whilst the portions of the photosensitive lacquer which lie beneath the opaque regions are not exposed to ultra-violet light and, during development, are removed to open the perforations in the screen. This produces a pattern on the screen of areas which transmit ink and areas that do not transmit ink.
Recently it has been proposed that, instead of using a photographic process to produce a pattern of ink transmitting and non-transmitting regions on the screen they are patterned using a laser engraving device to ablate the lacquer from regions of the screen that are required to transmit ink. Conventionally the perforated screen is slid onto a correspondingly sized metal mandrel which is then rotated about its longitudinal axis. A laser engraving head moves along parallel to the screen to ablate the lacquer from the perforations. One difficulty with this process is in removing the screen from the mandrel after it has been engraved. Debris from the engraving process which is blasted through the perforations tends to lock the screen onto the underlying mandrel.
In an attempt to overcome this difficulty and take up tolerances in the diameter typically of the order of 300 microns, an inflatable mandrel has been used in which a thin nickel sleeve is located around the outside of the mandrel and then the nickel screen slipped over the outside of the nickel sleeve. The space between the nickel sleeve and the mandrel is then inflated to expand the nickel sleeve and lock the screen in place. After the screen has been engraved, the screen is deflated and this allows sufficient clearance between the screen and the sleeve to enable the screen to be removed, even though debris has been blasted through the perforations in between the sleeve and the mandrel. This technique however suffers from the further difficulty that the screen is not supported concentrically whilst it is engraved and this means that the patterns are not recorded faithfully during the engraving of the printing screen.